Brake pedal bar to M/C

[GTVeloce]

Hi all

I have a clunk in my brake pedal in my 75 when I get about half way through the pedal travel which I would like to eliminate. It does not affect the pedal operation but it is a little disconcerting and must go. I have done some investigation and it appears to be coming from the rubber on the master cylinder side of the cross bar. I had six spacers/shims between the clamps and removed them on the weekend hoping that would remove the clunk. It doesn't appear to have done anything. The rubber looks to be in good condition.

So, two questions. Firstly, what does the spacers do exactly, and;
secondly, how do I eliminate this clunk?

Thanks
Julian